A percussion hammer of this nature is known from U.S. Pat. No. 4,450,920 and PCT/NO2012/050148. Further examples of prior art are shown in SE 444127B and U.S. Pat. No. 2,758,817A.
Hydraulically driven rig mounted percussion hammers for drilling in rock have been in commercial use for more than 30 years. These are used with joinable drill rods where the drilling depth is restricted by the fact that the percussion energy fades through the joints such that little energy finally reaches the drill bit.
Downhole hammer drills, i.e. hammer drills installed right above the drill bit, is much more effective and are used in large extent for drilling of wells down to 2-300 meter depth. These are driven by compressed air and have pressures up to approximately 22 bars, which then restricting the drilling depth to approximately 20 meters if water ingress into the well exists. High pressure water driven hammer drills have been commercial available more than 10 years now, but these are limited in dimension, as far as we know up to about 130 mm hole diameter. In addition, they are known to have limited percussion frequency, relatively low efficiency, and to have limited lifetime and are sensitive for impurities in the water. They are used in large extent in the mining industries since they are drilling very efficiently and drill very straight bores. They are used in a limited extent for vertical well drilling down to 1000-1500 meters depth, and then without any directional control.
It is desired to manufacture downhole drill fluid driven hammer drills which can be used together with directional control equipment, which have high efficiency, can be used with water as drill fluid and can also be used with water based drill fluid having additives, and having economical lifetime. It is expected great usage both for deepwater drilling for geothermic energy and for hard accessible oil and gas resources.
In percussion drilling, drill bits are used having inserted hard metal lugs, so called “indenters”. These are made of tungsten carbide and are typically from 8 to 14 mm in diameter and have spherical or conical end. Ideally viewed, each indenter should strike with optimal percussion energy related to the hardness and the compressive strength of the rock, such that a small crater or pit is made in the rock. The drill bit is rotated such that next blow, ideally viewed, forms a new crater having connection to the previous one. The drilling diameter and the geometry determine the number of indenters.
Optimal percussion energy is determined by the compressive strength of the rock, it can be drilled in rock having compressive strength over 300 MPa. The supply of percussion energy beyond the optimal amount, is lost energy since it is not used to destroy the rock, only propagates as waves of energy. Too little percussion energy does not make craters at all. When percussion energy per indenter is known and the number of indenters is determined, then the optimal percussion energy for the drill bit is given. The pull, or drilling rate, (ROP—rate of penetration) can then be increased by just increasing the percussion frequency.
The amount of drilling fluid pumped is determined by minimum necessary return rate (annular velocity) within the annulus between the drill string and the well bore wall. This should at least be over 1 m/s, preferably 2 m/s, such that the drilled out material, the cuttings, will be transported to the surface. The harder and brittle the rock is, and the higher percussion frequency one is able to provide, the finer the cuttings become, and the slower return rate or speed can be accepted. Hard rock and high frequency will produce cuttings that appear as dust or fine sand.
The hydraulic effect applied to the hammer drill is determined by the pressure drop multiplied with pumped quantity per time unit.
The percussion energy per blow multiplied with the frequency provides the effect. If we look into an imaginary example where drilling into granite having 260 MPa compressive strength and drilling diameter of 190 mm is performed, water is pumped by 750 l/min (12.5 liters/second) from the surface. It is calculated that approximately 900 J is optimal percussion energy.
With reference to known data for corresponding drilling, but with smaller diameters, a drilling rate (ROP) of 22 m/h (meters per hour) with a percussion frequency of 60 Hz, can be expected. It is here assumed to increase the percussion frequency to 95 Hz, consequently ROP then become 35 m/h. Required net effect on the drill bit then becomes: 0.9 Kj×95=86 kW. We assume the present hammer construction to have a mechanical-hydraulic efficiency of 0.89, which then provides 7.7 MPa required pressure drop over the hammer.
This hammer drill will then drill 60% quicker and by 60% less energy consumption than known available water propelled hammer drills.